Machine element

ABSTRACT

The invention relates to a machine element which is provided, at least over part of its axial length, with first toothing ( 2.1 ) in the form of threads, running on its cylindrical lateral surface, for a threaded drive. This is distinguished in that the threads are produced by means of a continuous shape rolling process in the axial direction, as is known per se, the continuous shape rolling process being used in a partially continuous manner corresponding to the desired axial extent of the threads.

FIELD OF THE INVENTION

The invention relates to a machine element which is provided, at leastover part of its axial length, with first toothing in the form ofthreads, running on its cylindrical lateral surface, for a threadeddrive.

BACKGROUND OF THE INVENTION

Machine elements of this type are used in a variety of ways in differentthreaded drives, for example for performing transport, steering orbraking functions.

A machine element of this type, embodied as a toothed rack for arack-and-pinion steering system, is already known from WO 02/076808 A1.In a power steering system of this type, the manual steering input isassisted by an electric motor which has a drive connection to thetoothed rack. The electric motor provides servo assistance to theconventional steering device where, in the usual way, a steering demandis transmitted from a steering wheel via a steering shaft to a pinionwhich acts on the toothed rack. A torque measuring device is installedbetween the steering shaft connection and the pinion. The torquemeasuring device senses the relative torque between the steering shaftconnection and the pinion and controls the drive of the electric motorby means of an electronic module. A ball screw is used to convert therotational movement of the electric motor into an axial movement of thetoothed rack.

The toothed rack shown in FIG. 2 of the cited prior publication for therack-and-pinion steering system of a motor vehicle has two toothedregions. It has both the flat toothing into which the toothed pinionengages and also toothing arranged on the lateral surface in the form ofthreads, into which the threaded nut, which is connected to the electricmotor, engages.

It is not possible on account of the different surface geometries andthe differently designed toothing regions to produce toothed racks ofthis type by continuous shape rolling and then simply cutting to adesired length.

SUMMARY OF THE INVENTION

The object on which the invention is based is therefore that ofcost-effectively producing a machine element of a generic embodiment ofthis type.

According to the invention, this object is achieved as claimed in thecharacterizing part of claim 1, in conjunction with its preamble, inthat the threads are produced by means of a continuous shape rollingprocess in the axial direction, as is known per se, the continuous shaperolling process being used in a partially continuous mannercorresponding to the desired axial extent of the threads.

This means that the relevant region of the machine element is placed atleast between two rotating shape rolling tools which are initiallyspaced apart from one another and are subsequently moved towards oneanother. As it rotates between the rotating shape rolling tools themachine element obtains the negative shape of said rolling tools, and ismoved little by little through the shape rolling tools in the axialdirection. Once the desired functional length of the machine element hasbeen provided with threads, the shape rolling tools are moved away fromone another and the machine element can be removed, a run-in region anda run-out region of the threads being present for production reasons.

The advantage of the solution according to the invention is that machineelements of this type having partial function areas can be produced in acost-effective manner by means of this non-cutting shaping process. Inparticular, no cutting pre-working or re-working processes are requiredin the toothed region. The axial extent of the thread profile can alsobe limited to the functionally required length by means of thispartially continuous shape rolling process, and this has an effect inparticular on the stability of machine elements of low diameter. Thepartially continuous shape rolling process has the additional advantagethat, in contrast to shape rolling using the single-pass process, asignificantly lower tool width is required and hence lower shapingforces are necessary. It is also advantageous that better toothingquality can be obtained using the partially continuous shape rollingprocess than when using the single-pass process.

Advantageous embodiments of the invention are described in the sublaims.

Claim 2 provides that the machine element is provided with secondtoothing which, according to claim 3, is of a flat form.

In a refinement of the invention, claim 4 provides that the machineelement is embodied as a single-part toothed rack for a rack-and-pinionpower steering system having an electric motor for a motor vehicle,which power steering system is operatively connected to steered wheels,a toothed pinion, which is connected to a steering wheel, engaging inthe first toothing of the toothed rack and, in order to convert therotational movement of the electric motor into an axial movement of thetoothed rack, the threaded drive being provided, whose threaded shaft,which has threads, is surrounded by a threaded nut, so that rollersarranged between said threaded shaft and said threaded nut roll both inthe threads of the threaded shaft and in threads of the threaded nut.

The invention is explained in more detail on the basis of the followingexemplary embodiment of a toothed rack for a rack-and-pinion steeringsystem of a motor vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a side view of a toothed rack having a threaded drive,

FIG. 2 shows a perspective illustration of the toothed rack,

FIGS. 3 a to 3 c show a schematic illustration of the profile of atoothed rack and

FIG. 4 shows a section of a rack-and-pinion power steering system for amotor vehicle.

DETAILED DESCRIPTION OF THE DRAWINGS

As can be seen from FIG. 4, a toothed rack 2 is mounted in a housing 1so as to be longitudinally displaceable and is connected, in a mannerwhich is not illustrated, at the left- and right-hand sides to asteering linkage which in turn is operatively connected to wheels(likewise not illustrated) which are to be steered. The toothed rack 2has a first toothing 2.1 whose threads are arranged on the lateralsurface of the toothed rack 2. As can be seen, the first toothing 2.1runs only over a partial axial region of the toothed rack 2. The powersteering system also comprises an electric motor 5 which is connected toa threaded drive 7 by means of a belt 6. The toothed rack 2 alsocomprises a second flat toothing 2.2 into which a toothed pinion 3engages which is connected to a steering shaft 4, which in turn is movedby a steering wheel (likewise not illustrated).

If the toothed pinion 3 is moved by the driver, via the steering wheeland the steering shaft 4, to the right or the left in thecircumferential direction, the relative torque between the steeringshaft 4 and the toothed pinion 3 is sensed by a torque measuring deviceinstalled in between the latter. The torque measuring device controlsthe drive of the electric motor 5, which is connected to the threadeddrive 7 by means of the belt 6. When the threaded nut of the threadeddrive 7 rotates, the toothed rack 2 is moved corresponding to the arrowsto the right or to the left in the axial direction, so that the desiredsteer angle of the wheels is set.

The toothed rack 2 illustrated in FIGS. 1 and 2 having the firstcircumferential toothing 2.1 in the form of threads arranged on thelateral surface and the second flat toothing 2.2 shows that, when thebelt pulley 7.1 of the spatially fixed threaded drive 7 rotates, thetoothed rack 2 is moved in the axial direction according to thedirection of rotation.

As the schematic illustrations in FIGS. 3 a, 3 b and 3 c show, at leasttwo shape rolling tools 8, 9 are provided which are spaced apart fromone another with their axes parallel. The shape rolling mill 8 is heldrotatably on a slide (not illustrated), while the shape rolling mill 9is rotatably mounted on another slide (likewise not illustrated). Thetwo slides can be moved toward one another and moved away from oneanother so that the size of the rolling gap can be set. The two shaperolling tools 8, 9, which are driven at the same speed, rotate in thesame direction, as indicated by their rotational direction arrows. As aresult, the continuous shape rolling process causes the toothed rack 2to rotate in the opposite direction. In this cold rolling process, thetoothed rack 2 is now placed into the working space, that is to say intothe closed rolling gap between the two shape rolling tools 8, 9, bymeans of a suitable supply device (not illustrated). When the shaperolling tools 8, 9 rotate, the toothed rack 2 is likewise set inrotation, and is pulled little by little through the two shape rollingtools 8, 9 in the axial direction. As can be seen in FIGS. 3 a to 3 c,the toothed rack 2 is moved through the shape rolling tools 8, 9 in thedirection of the arrow until the desired functional length of thetoothed rack 2 is provided with the circumferential first toothing 2.1.This functional length is then larger, as seen in the axial direction,than the axial width of the shape rolling tools 8, 9.

LIST OF REFERENCE SYMBOLS

-   1 Housing-   2 Toothed rack-   2.1 First toothing-   2.2 Second toothing-   3 Toothed pinion-   4 Steering shaft-   5 Electric motor-   6 Belt-   7 Threaded drive-   8 Shape rolling tool-   9 Shape rolling tool

1. A machine element which is provided, at least over part of its axiallength, with first toothing in the form of threads, running on itscylindrical lateral surface, for a threaded drive, characterized in thatthe threads are produced by means of a continuous shape rolling processin the axial direction, as is known per se, the continuous shape rollingprocess being used in a partially continuous manner corresponding to thedesired axial extent of the threads.
 2. The machine element as claimedin claim 1, characterized in that it is provided with second toothing(2.2).
 3. The machine element as claimed in claim 2, characterized inthat the second toothing (2.2) is of a flat form.
 4. The machine elementas claimed in claim 1, characterized in that it is embodied as asingle-part toothed rack (2) for a rack-and-pinion power steering systemhaving an electric motor (5) for a motor vehicle, which power steeringsystem is operatively connected to steered wheels, a toothed pinion (3),which is connected to a steering wheel, engaging in the second toothing(2.2) of the toothed rack (2) and, in order to convert the rotationalmovement of the electric motor (5) into an axial movement of the toothedrack (2), the threaded drive (7) being provided, whose threaded shaft,which has threads, is surrounded by a threaded nut, so that rollersarranged between said threaded shaft and said threaded nut roll both inthe threads of the threaded shaft and in threads of the threaded nut.